Although many nations of the world have joined in a treaty to preclude the use of lasers as anti-personnel weapons, some of the more radical nations in the world may yet decide to use laser radiation as a weapon against individuals. There are numerous laser-operated weapon guidance devices, including target designators in which the target, such as an aircraft, is "painted" with laser radiation, and a guidance system homes in on that radiation. In either of these cases, it is quite possible that humans will be subjected to laser radiation sufficient to damage the eye. Furthermore, the onset of the laser radiation may be very sudden, giving insufficient time to prepare for it. In order to quantify, to some extent, the effects of high intensity laser radiation on human eyes, some of the following analysis utilizes data related to rabbits and monkeys presented by Birngruber et al, "Femtosecond Laser-Tissue Interactions: Retinal Injury Studies", IEEE Journal of Quantum Electronics, Volume QE-23, No. 10, October 1987, pp. 1836-1844. It is believed that the eye has an optical gain of anywhere from 20,000 to 100,000; that is, the intensity of radiation entering through the iris is amplified 20,000 to 100,000 times as it is focused on the retinal spot at the back of the eye. The retinal spot is on the order of 50 microns in diameter, or smaller, and it is understood that laser intensity of wavelengths between 4 K Angstrom and 14 K Angstrom of on the order of 2,000 watts per square centimeter, or more, will burn the retinal spot; that is, will permanently damage the retina to preclude vision. The human eye has a protection reflex which operates in about one-quarter second. Either the eye will close or the head will turn away from the irritating radiation in about that length of time. Therefore, one could anticipate that a weapon intentionally designed to blind personnel would have a wavelength which the eye will not respond to (that is, not between 8 K Angstroms and 14 K Angstroms) or if having a visible wavelength, will have suitable power to do all the necessary damage in less than one-quarter second. FIG. 12 of Birngruber, et al describes the dependence, in rabbits and monkeys, of retinal injury threshold upon laser pulse duration. That data, however, relates to intensity of radiation at the retina; due to the high gain of the eye, the intensity of radiation outside the eye to achieve the data referred to in the figure is less by a factor of between 20,000 and 100,000. Therefore, radiation of on the order of, say, thirty milliwatts per square centimeter entering the iris, over a duration as small as one microsecond, is sufficient to reach the threshold of permanent damage at the retina. This compares with one milliwatt per square centimeter which is deemed unsafe; that is, it is painful but not inducing permanent damage (American Standard Institute ANSI Z136.1-1993).
It is understood that all eye protection known to the art simply utilizes attenuation, in many cases wavelength selective attenuation, to tend to protect eyes while still permitting the eye to see something of interest. In the case of laser radiation discussed hereinbefore, the degree of attenuation in the visible range would have to be sufficiently great so as to totally preclude any ordinary, ambient light reaching the eye in the absence of the radiation from a laser. Of course, totally filtering, by attenuation, radiation outside the visible range would still leave the eye subject to intentional destruction by lasers in the visible range.
The foregoing analysis is applicable to non-living optical systems which have extremely high gain within their optical receiving systems. This includes a variety of instruments, such as satellite surveillance systems. As used herein, the terms "high gain optics" and "high gain optical systems" include the human eye or eyes, and instruments which have significant optical gain and thereby may take advantage of the present invention.
Further, in the case of search or guidance instruments which are protected by narrow band filters having a center wavelength at the expected wavelength of a countermeasure of some sort, the countermeasure can switch wavelengths quite easily (such as by use of various isotopes of the lasing medium), thereby to mitigate the effectiveness of the filter. It is also known that a laser receptor protected by attenuators with narrow wavelength bands, in order to "see" in other portions of the visible spectrum, can be countermeasured easily, because all lasers can be shifted in wavelengths by large percents by various means such as isotopes of the lasing media, and therefore negate the attenuation protection.
In my prior U.S. Pat. No. 5,831,769, protection from laser pulses of greater than about one microsecond is provided.
Because thermal blooming requires a certain amount of time, the effective defocusing will occur only for laser radiation greater than about one microsecond duration, including continuous wave.